/Engine/Source/Editor/StaticMeshEditor/Public/StaticMeshEditorSubsystem.h
Remove then add LODs on a static mesh. The static mesh must have at least LOD 0. The LOD 0 of the static mesh is kept after removal. The build settings of LOD 0 will be applied to all subsequent LODs.
the number of LODs generated on the input mesh. An negative value indicates that the reduction could not be performed. See log for explanation. No action will be performed if ReductionOptions.ReductionSettings is empty
int32 SetLodsWithNotification ( [UStaticMesh](API\Runtime\Engine\Engine\UStaticMesh) * StaticMesh, const [FStaticMeshReductionOptions](API\Editor\StaticMeshEditor\FStaticMeshReductionOptions) & ReductionOptions, bool bApplyChanges )